The CYGNET Study (Cohort study of Young Girls'Nutrition, Environment, and Transitions) is a prospective cohort study of 444 girls that is examining environmental, lifestyle, and genetic factors in the development of early puberty and other hallmarks of maturation. Participants were aged 6-8 years at the time of enrollment from June 2005 to August 2006, and the investigators are currently in their 5th annual (4th follow-up) exam cycle. The study rationale lies in the growing recognition that the pubertal transition may be an important window of susceptibility in establishing long-term risk of breast cancer. The potential exposures to chemicals that are hormonally active raise concerns that these exposures and other lifestyle and genetic factors may play important roles in modifying age-at-onset of puberty and other hallmarks of sexual maturation. This cohort study is thus focused on determining factors that are associated with various milestones in sexual maturation. A key focus has been on breast development, as assessed by Tanner stage at annual clinical exams. As they continue to follow study participants, other parameters that may be associated with breast cancer risk are of interest, including onset of menses, establishment of cycle regularity, tempo (the time from onset of puberty to onset of menses), peak height velocity, and attainment of adult height. Exposures of interest are assessed through clinical exam (e.g., anthropometry), interviews and questionnaires (e.g., personal care product use, physical activity, food intake, psychosocial factors), and collection and assay of biospecimens (urine, blood, saliva) for environmental and genetic factors. Interactions with study participants and the broader community are facilitated by close integration with community partners, who are also involved in all aspects of study protocol. Through the 4th annual exam cycle, the investigators collected data from 391 (88%) of the original cohort of 444 girls. After the 1st follow-up year, they have retained annually over 97% of the cohort. Assuming similar annual retention rates, they estimate that for the ninth exam cycle in 2013-2014 when girls are 14-16 years old, we will see approximately 339 girls. By then, we expect that all girls will have begun puberty, most will have experienced peak height velocity and onset of menarche, and a small number will have achieved their adult height. Using conservative assumptions, the investigators should have adequate power to detect associations for most major endpoints of about 1.57 comparing high and low quartiles of a continuous variable such as biomarker levels. The investigators anticipate continuing our collaborations with similar studies in Cincinnati and New York.